X or lightdm doesn't start while external monitor is connected

When I start my laptop with external monitor connected, the boot process stops before lightdm would appear. I can not switch to any tty, they probably don’t exist. The only thing I can see is the big Dell logo.
But if I start without the external monitor, everything goes well and I can connect the monitor later.
I am using Manjaro XFCE with UEFI on a Dell laptop with Intel processor and integrated graphics card.
Is there anyone with any idea what could cause this weird behavior? Thanks for your help.

Please post some information about your setup as it might help others to assist you.